1. Field of the Invention
This invention relates to an energy subtraction processing method. This invention also relates to a stimulable phosphor sheet which is used for the energy subtraction processing method.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals which represent the image information recorded at corresponding sampling points in the radiation images are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy levels. Specifically, an object is exposed several times to radiation with different energy levels, and a plurality of radiation images are thereby obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
With the energy subtraction processing method proposed in U.S. Pat. No. 4,896,037, the operation for recording a radiation image is carried out twice with two kinds of radiation different in energy level, the two radiation images thus recorded are read out, and two digital image signals are thereby obtained. A subtraction process is then carried out on the digital image signals. However, with the proposed method, because a certain length of time occurs between the two operations for recording the radiation images, there is the risk that the object moves during the time interval. As a result, the two radiation images thus recorded do not coincide with each other, and a motion artifact occurs in the visible image reproduced from a difference image signal, which is obtained from the subtraction processing carried out on the image signals representing the two radiation images. Thus a visible image having good image quality cannot be obtained.
Also, even when the object does not move during the two operations for recording the radiation images, troublesome operations are required for the positions of the two radiation images to be adjusted quickly and accurately so that the image signal components of the image signals can be found which represent the image information recorded at corresponding sampling points in the radiation images.
U.S. Pat. No. 4,855,598 discloses an energy subtraction processing method wherein two radiation images to be subjected to the energy subtraction processing are recorded with a single recording operation. With the disclosed method, for example, a filter which exhibits different levels of absorptivity with respect to different radiation energy levels is located between two stimulable phosphor sheets, and the two stimulable phosphor sheets are simultaneously exposed to radiation which has passed through an object. With the disclosed method, no motion artifact due to movement of the object occurs in the visible image reproduced from a difference image signal, which is obtained from the subtraction processing carried out on the image signals representing the two radiation images. However, as in the aforesaid energy subtraction processing method wherein two operations for recording the radiation images are carried out, two stimulable phosphor sheets are used. Therefore, troublesome operations are required for the positions of the two radiation images to be adjusted quickly and accurately so that the image signal components of the image signals can be found which represent the image information recorded at corresponding sampling points in the radiation images. Also, with the disclosed method, the energy subtraction processing cannot be carried out with a high operating efficiency. Specifically, the two stimulable phosphor sheets and a filter located therebetween are housed in a cassette and subjected to the recording operation. After the radiation images have been stored on the two stimulable phosphor sheets but before they are read out from the two stimulable phosphor sheets, the two stimulable phosphor sheets must be taken out of the cassette and respectively loaded into two new cassettes in a darkroom, so that the read-out operation can be carried out on each of the two stimulable phosphor sheets. Such loading operations are troublesome. The loading operations can be carried out automatically. However, in order for the loading operations to be carried out automatically, complicated apparatuses or specific cassettes are necessary.
Novel energy subtraction processing methods have also been proposed in, for example, Japanese Unexamined Patent Publication Nos. 61(1986)-251799 and 61(1986)-251800. With the proposed methods, a single stimulable phosphor sheet is used which is provided with a plurality of stimulable phosphor layers capable of being stimulated with stimulating rays having different wavelengths or capable of emitting light having different wavelengths. However, the proposed methods have the drawbacks in that complicated systems are necessary because, for example, the read-out operation must be carried out on both surfaces of the stimulable phosphor sheet.